1. Field of the Invention
This invention relates to dielectric thin film elements, particularly dielectric oxide thin film elements, and also to thin film devices using the dielectric thin film elements. The invention also relates to a method for making such dielectric thin films as mentioned above. The thin fim devices of the invention may be, for example, a thin film electroluminescent device and a thin film transistor. The thin film electroluminescent device is suitably used in a flat display such as, for example, a character or graphic terminal display such as of personal computers and can thus be widely utilized in the field of office machines. The thin film transistor may be used to drive a light-receiving element such as in a pyroelectric IR detector or a light-receiving element such as in an electroluminescent display device.
2. Description of the Prior Art
The electric performance of dielectric thin films is determined by several factors or parameters such as a dielectric constant, a dielectric loss, a breakdown electric field strength, a breakdown mode and the like. These factors are all important, but when an electric capacitance and a dielectric strength are the first importance as in capacitors, the figure of merit, q.sub.m, is represented by the following equation EQU q.sub.m =.epsilon..sub.r .epsilon..sub.o E.sub.b
in which .EPSILON..sub.0 is a dielectric constant of vacuum of 8.854.times.10.sup.-12 F/m, .epsilon..sub.r and E.sub.b are, respectively, a specific inductive capacity and a breakdown electric field strength. The figure of merit, q.sub.m, is a maximum charge density (c/m.sup.2) at the time of dielectric breakdown. The figures of merit of various known dielectric thin films are shown in Table 1 below.
TABLE 1 ______________________________________ Figure of Merit, q.sub.m, E.sub.b Material (C/m.sup.2) (MV/cm) .epsilon.r ______________________________________ Y.sub.2 O.sub.3 0.03.about.0.05 3.about.5 12 Al.sub.2 O.sub.3 0.04 5 8 SiO.sub.2 0.02 6 4 Si.sub.3 N.sub.4 0.04.about.0.06 6.about.8 8 Ta.sub.2 O.sub.5 0.03 1.5 23 Ta--O--N 0.06 3.3 22 PbTiO.sub.3 0.07 0.5 150 SrTiO.sub.3 0.19.about.0.25 1.5.about.2 140 BaTa.sub.2 O.sub.6 0.07 3.5 22 PbNb.sub.2 O.sub.6 0.06 1.5 41 ______________________________________
As will be seen from the above table, perovskite type oxides, such as SrTiO.sub.3 and PbTiO.sub.3, have larger figures of merit than the other materials when applied as a dielectric thin film. This is ascribed to the large specific inductive capacity of these perovskite type oxides, but the breakdown electric field strength is relatively low.
On the other hand, it is known that tantalum metal used as a target is subjected to a reactive sputtering technique using a mixed gas of oxygen and nitrogen to obtain a dielectric oxide, such as Ta--O--N, whose breakdown electric field strength is two or more times the strength of tantalum oxide [S. J. Ingrey, W. D. Westwood, B. K. Maclaurin; Thin Solid Films 30 (1975) 377-381].
Upon determination of a Si--Al--O--N composite film obtained from a composite target of Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 by sputtering, the figure of merit has been found to be 0.064 C/m.sup.2, which is smaller than the values of the perovskite type oxide dielectric films. For the fabrication of thin film capacitors, if the breakdown electric field strength of the perovskite type oxide dielectric films can be increased to such an extent as other dielectric films, the figure of merit can be further improved, making it possible to make a capacitor of a large capacitance and a high breakdown voltage.
Moreover, if the perovskite type oxide dielectric film, which has a high breakdown electric filed strength, is provided, the film is useful in making thin film devices including, for example, thin film electroluminescent devices and thin film transistors. As is known in the art, an elctroluminescent device (hereinafter referred to simply as EL device) which emits light by application of an AC electric field includes a phosphor layer, a dielectric layer formed on one or both surfaces of the phosphor layer, and two electrodes formed on opposite sides of the phosphor layer. The phosphor layer is generally made of a matrix of ZnS, ZnSe or SnF.sub.2 to which Mn or fluorides of a rare earth metal serving as light-emitting centers are added. The dielectric materials used include Y.sub.2 O.sub.3, SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and Ta.sub.2 O.sub.5. In recent years, attempts have been made to employ perovskite type oxides such as, for example, PbTiO.sub.3, SrTiO.sub.3 and BaTiO.sub.3. In the electroluminescent device, the phosphor layer has generally a thickness of from 500 to 700 nm and the dielectric layer has a thickness of from 400 to 800 nm. With a ZnS phosphor device in which Mn is added as light-emitting centers, a maximum luminance reaches 3500 to 5000 Cd/m.sup.2 by voltage application at a frequency of 5 KHz.
When the device is energized by AC current, the voltage applied to the device is divided into the phosphor layer and the dielectric layer. The EL device is equivalent to two capacitors connected in series, so that EQU .epsilon..sub.i V.sub.i /t.sub.i =.epsilon..sub.Z V.sub.Z /t.sub.Z
in which and .epsilon..sub.i, V.sub.i, t.sub.i are, respectively, a specific indictive capacity, an applied voltage and a thickness with respect to the dielectric layer, and .epsilon..sub.Z, V.sub.Z t.sub.Z are, respectively, a specific inductive capacity, an applied voltage and a thickness with respect to the ZnS. If the thicknesses of the respective layers are assumed to be equal, .epsilon..sub.Z is about 8 to 9 for ZnS, so that with Y.sub.2 O.sub.3, SiO.sub.2 Si.sub.3 N.sub.4 and Ta.sub.2 O.sub.5 whose .epsilon..sub.i is in the range of about 4 to 25, only a half of the external voltage is applied to the ZnS layer. On the other hand, the perovskite type oxides, such as, for example, PbTiO.sub.3, BaTiO.sub.3 and SrTiO.sub.3, have an .epsilon..sub.i value of about 50 to 150, and thus about 80% of the external voltage is applied to the ZnS layer. In this sense, the perovskite type oxides are useful as a dielectric layer for the EL device. However, the perovskite type oxide layer is disadvantageous as described before, i.e. the breakdown electric field strength is below half the strength of thin films of low dielectric constants such as of Y.sub.2 O.sub.3. For instance, with PbTiO.sub.3, the strength is about 0.5 MV/cm and with SrTiO.sub.3, the strength is about 1.5 to 2 MV/cm. On the other hand, with Y.sub.2 O.sub.3, the strength is about 3 to 5 MV/cm and with Si.sub.3 N.sub.4, the strength is from 6 to 8 MV/cm.
The above disccusion is true of a thin film transistor which is used to drive a light-receiving device, such as a pyroelectric detector, or a light-emitting device, such as an EL display.